When i first started reading up on the Linksys WRT54Gx series I came across a post from someone in Oz whod set up a connection using directional antenna over a considerably bigger distance than that. The point being all it took was a cheap router, a custom firmware and a bit of time and a few parts so it is doable.
Google threw up
http://www.wilac.net/descargas/documentos/EnlaceAguila_Baul_EN.pdf